The invention relates to a method of recovering linear polyester, such as PET and PBT, from the waste of same, and a device for carrying out the method.
Unlike many other meltable thermoplastic plastics such as polyethylene, polypropylene or polystyrene, linear polyesters such as polyethylene terephthalate cannot be processed by simple melting and regranulation to make a reusable raw material for higher quality requirements, because polyesters are hydrolytically degraded during melting by the moisture which always adheres, unless they have been previously dried. On the other hand the breakdown of the polymer chains by hydrolysis is reversible on polyesters, by polycondensation being carried out through suitable reaction conditions such as vacuum, high temperature and stirring, and the reaction water formed being removed. In contrast to the other plastics mentioned, in polyesters even the breakdown of the molecule chains caused by thermal or oxidative degradation can be repaired.
Polyethylene terephthalate (PET) as waste is frequently marked by an increased concentration of carboxyl terminal groups in relation to intact PET and a depletion of glycol ester terminal groups. The cause of this is first and foremost the thermal degradation which is unavoidable during melting and during the processing from the melt. This leads to the fact that in usual regranulating processes the chain length of the PET, measured by the intrinsic solution viscosity (i.v.) decreases; if the behaviour of the PET in repeated regranulation and reuse is observed, as can be expected in intensive recycling, it can be ascertained that after roughly the third regranulation, the intrinsic solution viscosity has sunk so far, that the material is unusable. The intrinsic solution viscosity can be raised again by solid-phase postcondensation of the PET, this measure coming up against limits, however, since the carboxyl terminal groups have in the meantime increased and the glycol ester terminal groups have decreased. The greater the COOH/OH ratio, the lower the intrinsic solution viscosity which may be achieved through postcondensation, and the higher the COOH concentration of the postcondensation product.
Methods of recycling PET are known, in which the irregular waste is crushed and compacted in such a way that the parts of the waste so treated can be led continuously through shaft or other dryers, in order to achieve complete drying of the parts. Thereafter the melting and postcondensation are carried out. This method has the disadvantage that the drying of the waste is very expensive as a result of the extent of the plant and the energy consumption, and that hydrolytic degradation cannot be completely avoided, even if the waste is dried.
With these methods, the degree of polycondensation which may be obtained in the end product depends on the quality of the polyester waste used as raw material, in particular on its intrinsic solution viscosity (i.v.) and its COOH concentration. The end product from this method is therefore excluded from many applications which require a certain degree of polycondensation. Such products must therefore be described as inferior.
These methods are moreover not in the position to compensate for the loss of glycol ester terminal groups which increases with time and which is unavoidable in the repeated recycling of polyester, especially in the practically closed circuit.
The purpose underlying the invention is to create a method and a device for recovering linear polyester with simultaneous hydrolytic and glycolytic degradation, by means of which a recycled polyester of high quality should be obtained, and so that recycling is possible even in the closed circuit, it being intended that the plant size and the energy consumption should be kept small. In particular, it is the purpose of the invention to produce a polyester with exactly the degree of polycondensation which is necessary for the respective application, independently of the quality (i.v., concentration of carboxyl terminal groups) of the raw polyester.